In many industries, such as the aerospace industry, one of the effective ways to reduce weight of an aircraft is to reduce the density of aluminum alloys used in the aircraft's construction. It is known in the art that aluminum alloy densities may be reduced by the addition of lithium. However, lithium in aluminum-based alloys also raises other problems. For example, the addition of lithium to aluminum alloys may result in a decrease in ductility and fracture toughness. For use as aircraft structural parts, it is obviously imperative that any alloy have excellent fracture toughness and strength properties.
Various aluminum-lithium alloys have been registered with the Aluminum Association. For example, alloys AAX2094 and AAX2095, registered in 1990, include alloying elements of copper, magnesium, zirconium, silver, lithium and inevitable impurities. U.S. Pat. No. 5,032,359 entitled "Ultra High Strength Weldable Aluminum-Lithium Alloys" issued Jul. 16, 1991, discloses an improved aluminum-copper-lithium-magnesium-silver alloy possessing high strength, high ductility, low density, good weldability and good natural aging response. Typically, these alloys consist essentially of 2.0-9.8 wt.% of an alloying element which may be copper, magnesium, or mixtures thereof, the magnesium being at least 0.01 wt.%, with about 0.01-2.0 wt.% silver, 0.05-4.1 wt.% lithium, less than 1.0 wt.% of a grain refining additive which may be zirconium, chromium, manganese, titanium, boron, hafnium, vanadium, titanium di-boride or mixtures thereof.
Another prior art alloy for use in aircraft industry application is disclosed in U.S. Pat. No. 4,648,913 to Hunt, Jr. et al. In this patent, an aluminum-based alloy is disclosed comprising 0.5-4.0 wt. % lithium, 0-5.0 wt. % magnesium, up to 5.0 wt.% copper, 0-1.0 wt.% zirconium, 0-2.0 wt.% manganese, 0-7.0 wt.% zinc, 0.5 wt.% maximum iron, 0.5 wt.% maximum silicon, the balance aluminum and incidental impurities. This alloy is subjected to heat treating and working steps to improve strength and toughness characteristics.
Despite the years of developmental effort, these aluminum-lithium alloys have been selected for relatively few commercial applications. One of the reasons for such a limited commercial success of these aluminum-lithium alloy products is severe strength anisotropy of highly wrought products in high strength T8 temper conditions.
The T8 temper designation, as is well known to those skilled in the art, includes solution heat treatment, strain hardening and then artificial aging.
Subjecting aluminum-lithium alloys to conventional T8 temper practice results in a wide variation in strengths at different thickness locations and in different directions for a given wrought product. For example, the tensile yield strength of a given product can vary up to almost 20 ksi between different thicknesses and locations in the wrought product.
In the aforementioned Hunt, Jr. et al. patent and related U.S. Pat. Nos. 4,797,165 and 4,897,126 to Bretz et al. and 4,961,792 to Rioja et al., solution heat treatment, stretching and aging steps are disclosed to improve strength and toughness in aluminum-lithium alloys. In the Rioja et al. patent, stretching or equivalent working following the solution heating step is disclosed as greater than 1% and less than 14%. However, the Rioja et al. patent and the patents related thereto fail to address the deficiency in aluminum-lithium alloys regarding strength anisotropy.
As such, a need has developed to provide improved processing techniques to achieve high strength and minimize strength anisotropy to facilitate full commercial implementation of aluminum-lithium alloy wrought products.
In response to this need, the present invention provides a method of improving strength anisotropy in aluminum-lithium alloys by imparting a sequence of cold rolling and stretching steps between the solution heat treating steps and aging steps used in T8 temper practice. None of the prior art discussed above teaches or fairly suggests minimizing strength anisotropy in aluminum-lithium alloys by modifying the T8 temper practice.
Although the aforementioned patents to Bretz et al., Rioja et al. and Hunt, Jr. et al., indicate that improvement in strengths are achieved in aluminum-lithium alloys using the disclosed solution heat treatment, stretch and aging processing, these improvements in strength do not extend to all thickness locations and all directions for a given wrought product. As will be demonstrated hereinafter, the prior art stretching techniques generally provide only improvements in strength in the T/2 location and longitudinal direction. Tensile yield stresses in other locations and directions, e.g., the T/8 location and 45 degree direction, have drastically reduced strengths. Since commercial applications are based upon design requirements which must adhere to the lowest level of strength for a given alloy, the failure to increase strengths in locations and directions other than the T/2 and longitudinal direction limits the commercial acceptance of these types of alloys.